In cellular network optimization, one common task is the so-called propagation model optimization, which customarily requires at least 30 dB of dynamic range of detection. Historically, this and higher levels of dynamic range were achieved by using frequency separation between signals, such as in frequency division multiplex (FDM) systems or by using special transmitters in “key-up” measurement campaigns. Later, the dynamic range of the measurements in CDMA-based networks was increased when the signals themselves were designed with the goal of increased detection ability. In particular, extremely long pseudo random codes were used in CDMA and WCDMA-based protocols, which enabled detection of signals buried under more than 30 dB of interference and noise. However, the exigencies of better protocol efficiencies made the design of newer, OFDMA-based signals less effective for detection of weak signals in the presence of stronger interference. For example, the detection of WiMAX signals using the so-called preamble, which carries information about the source sector, becomes problematic at close to −13 dB level.
With the advent of MIMO technologies in the fourth generation of cellular networks, the radio receivers for signal detection and measurements (“scanners”) will be built as multichannel coherent parallel receivers in order to provide measurements of channel characteristics pertinent to MIMO capacity of radio channels. It is possible to use this multichannel architecture of the receivers, together with a multi-antenna array, to also improve the ability to discriminate between signals and improve the dynamic range by using such known techniques as beam-forming and interference cancellation.
Despite recent advances of the computer technology, building a practical multichannel detection and measurement receiver for modern 4-G technologies like WiMAX and LTE presents a definite challenge. For example, for the widest standardized LTE signal bandwidth of 20 MHz one would need to digitize and store two signal samples at the rate of more than 20 Msps (samples per second) or more realistically, at 30 Msps. At 16 bits per sample (2 bytes), that amounts to 30×2=60 Mbytes/sec per antenna. It is desirable to have 8 antennae in an array, which yields 8×60=480 Mbytes/sec for the system. Although this is feasible with modern serial busses, the storage requirements are pushing the envelope since the recording has to continue for hours in a usual drive-testing scenario. In one hour, the system will fill 480×3600=1.728 TB of memory. After eight hours of driving it will come to 13.8 TB, which clearly exceeds the practical limit of current storage technology. It is desirable to keep this requirement under 2 TB, which is the size of widely available hard disks today.
In addition to the limitations of the storage technology, a massively multichannel radio receiver is prohibitively expensive and power-hungry. There is a need to decrease the number of RF channels in systems for signal detection and measurement without appreciably sacrificing their characteristics.